Download Axial ratio improvement in aperture antennas using high

Axial ratio improvement in aperture
antennas using high-impedance
ground plane
D. Sievenpiper, J. Schaffner and J. Navarro
High-impedance surfaces are studied as a means of improving the
radiation pattern symmetry in aperture antennas. Compared to a solid
metal ground plane, the results suggest that high-impedance surfaces
can significantly improve the axial ratio of circularly polarised
antennas. This can be used to reduce interference between left and
right polarisation components.
Introduction: Many communication systems use circular polarisation. However, the task of designing an antenna that transmits or
receives in circular polarisation over a wide range of angles is often
complicated by the presence of the metallic structures on which the
antenna is mounted. For example, antennas on a flat metal ground
plane will tend to emit in vertical polarisation at low angles, because
horizontal fields are shorted by the metal surface, while vertical fields
can propagate along the metal. We often describe the polarisation
purity of a wave in terms of its axial ratio, which is the ratio of the
major axis to the minor axis of the polarisation ellipse.
It is known that a variety of surface textures can improve the radiation
characteristics of antennas. Soft and hard surfaces [1] are often used to
alter the electromagnetic boundary condition of a metal surface, to
either suppress or enhance surface waves of either polarisation. These
typically consist of corrugations running either transverse or longitudinal to the direction of propagation across the surface. For example,
a radially symmetric soft surface has been shown to reduce the axial
ratio of various kinds of circularly polarised antennas [2]. However, for
arrays or other complex antennas, where one may want to surround
several separate radiators with such a material, it may not be possible to
use a structure with only radial symmetry. Another candidate is a twodimensionally periodic structure called the high-impedance surface.
High-impedance surface: By covering a metal sheet with a resonant
surface texture, we can alter its electromagnetic surface impedance,
and also its surface wave properties. One such texture is a twodimensionally periodic structure often known as a high-impedance
surface [3]. It consists of an array of small metal patches connected to
the metal surface by conducting posts. It is often built using printed
circuit board techniques. An example of such a structure is shown in
Fig. 1. The surface can be modelled as a resonant LC circuit, where
the proximity of the metal patches provides capacitance, and the
conductive path between them provides inductance. Near the resonance frequency, the impedance of the surface is high compared to the
impedance of free space, and the surface has a reflection phase of 0,
compared to a flat metal surface with a reflection phase of p. For the
surface studied in this Letter, we used metal patches 3.2 mm2, and
with a lattice constant of 3.7 mm. The circuit board was made of
Rogers Duroid 5880, and was 1.57 mm thick. These dimensions
provide a resonance frequency near 15 GHz.
The resonance frequency lies within a surface wave bandgap, where
surface waves of both TM and TE polarisation are suppressed. An
example of this suppression is shown in Fig. 2. Two small coaxial
probes were placed near the surface, and the transmission between them
was measured. Below the resonance frequency, the surface is inductive,
and it supports TM surface waves. Above the resonance frequency, the
surface is capacitive and supports TE surface waves. Between these two
regions is a bandgap, centred about the resonance frequency, which
spans from roughly 12 to 18 GHz. At the upper edge of the bandgap,
we see the onset of leaky TE waves as a soft edge in the transmission
plot. These leaky waves can be used for horizontally polarised antennas
that radiate at low angles. Closer to the centre of the bandgap, both TM
and TE waves are strongly suppressed, since the leaky TE waves radiate
in the normal direction from the surface. In the region where both
polarisations are suppressed, this textured surface can be used to make
antennas with very symmetric radiation patterns, compared to a flat
metal surface which only supports TM waves, while suppressing
TE waves.
Fig. 2 Measured transition magnitude between two small coax probes
located near surface
Antenna measurements: We built a simple aperture antenna, shown in
Fig. 1, to demonstrate the use of the high-impedance surface as a
means of improving the symmetry of the radiation pattern. The
aperture was the open end of a standard Ku-band rectangular waveguide, which was attached to a similarly sized rectangular hole in the
centre of a 12.7 cm square high-impedance surface. For comparison,
we also made an identical antenna with a metal ground plane of the
same size.
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E-plane
H-plane
Fig. 3 Radiation pattern of aperture antenna in conventional metal
ground plane
a
b
Fig. 1 Side and front view of aperture antenna in high-impedance surface
a Side view
b Front view
While the radiation pattern is somewhat affected by the shape of the
aperture, it is primarily determined by the geometry of the surrounding
ground plane, and the electromagnetic boundary condition of that
surface. The flat metal ground plane supports the propagation of TM
polarised waves, because in these waves the electric field is perpendi-
ELECTRONICS LETTERS 7th November 2002 Vol. 38 No. 23
cular to the metal surface. Waves with this polarisation can propagate
for long distances in close proximity to a metal surface. For this reason,
the E-plane radiation pattern in Fig. 3 is quite broad. TE waves, on the
other hand, cannot propagate at grazing angles to a metal surface
because their transverse electric field is shorted by the conducting
surface. The H-plane is therefore much narrower. This is the expected
radiation pattern for an aperture antenna in a conducting ground plane.
On the textured ground plane, the pattern is much more symmetrical,
as shown in Fig. 4. This can be attributed to the suppression of both TM
and TE surface waves near the resonance frequency. The gain is also
higher in the forward direction, and this can be attributed to standing
waves that occur at the resonance frequency and surround the aperture,
which cause a slight increase the effective aperture area. The radiation
patterns shown here are for 13 GHz. The antenna produces a similar
pattern throughout most of the bandgap region. However, as the
frequency is increased toward the upper edge where leaky TE waves
are supported, the H-plane actually becomes broader than the E-plane.
If one had a ground plane that behaved as a magnetic conductor, one
would expect a broad H-plane and a narrow E-plane—the opposite of
the electric conductor shown in Fig. 3. Thus, in this way the textured
surface mimics a magnetic conductor.
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5 September 2002
D. Sievenpiper and J. Schaffner (HRL Laboratories LLC, 3011 Malibu
Canyon Road, Malibu, CA, 90265, USA)
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Conclusion: By surrounding an antenna with this textured highimpedance ground plane, the radiation pattern symmetry is significantly improved over a broad bandwidth centred at the resonance
frequency of the surface. This suggests that for a circularly polarised
antenna, a textured ground plane can provide an improvement in axial
ratio, and a reduction in interference from the opposite polarisation.
We have also found that in the upper portion of the bandgap, the
H-plane radiation pattern is actually broader than the E-plane, due to the
onset of leaky TE waves above the resonance frequency. This
corresponds to the expected radiation pattern for an artificial magnetic
conductor.
# IEE 2002
Electronics Letters Online No: 20020984
DOI: 10.1049/el:20020984
330
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linearly polarised antenna, the results can be extrapolated to circularly
polarised antennas, which can be represented using two orthogonal
linear antennas that are out of phase by 90 . In this case, the improved
symmetry of the radiation pattern translates into an improvement in
axial ratio. For angles greater than 75 from normal, the improvement is
greater than 10 dB over a broad frequency range. For example, at 60 ,
an antenna on the metal ground plane would have an axial ratio of no
better than 8 dB, while an antenna on the textured ground plane could
have an axial ratio of close to 0 dB.
J. Navarro (The Boeing Company, PO Box 3999 MC3W-51, Seattle,
WA, 98124, USA)
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References
–25
270
90
E-plane
H-plane
Fig. 4 Radiation pattern of aperture antenna in high-impedance surface
Regardless of the leaky TE waves, the E-plane and H-plane patterns
are much more similar for the textured surface over much of the
bandgap. While these simple experiments were performed using a
1
2
3
KILDAL, P.-S.: ‘Artificially soft and hard surfaces in electromagnetics’,
IEEE Trans. Antennas Propag., 1990, 38, pp. 1537–1544
YING, Z., and KILDAL, P.S.: ‘Improvements of dipole, helix, spiral,
microstrip patch and aperture antennas with ground planes by using
corrugated soft surfaces’, IEE Proc., Microw., Antennas Propag., 1996,
143, pp. 244–248
SIEVENPIPER, D.: ‘High-impedance electromagnetic surfaces’. PhD
Dissertation, Department of Electrical Engineering, University of
California, Los Angeles, CA, 1999
ELECTRONICS LETTERS 7th November 2002 Vol. 38 No. 23